The invention relates to a capacitor in an integrated circuit having a noble-metal-containing first electrode, in which a high-∈ dielectric material or a ferroelectric material is used as the capacitor dielectric.
Capacitors are required in a large number of semiconductor integrated circuits, for example in DRAM circuits or A/D converters. A primary aim in this context is to increase the integration density, i.e. it is necessary to achieve as high a capacitance as possible, or a capacitance which is sufficient for the requirements, with a minimum amount of space. The problem presents itself in particular in DRAM circuits, in which each memory cell has a storage capacitor and a selection transistor, while the area available for a memory cell is being constantly reduced. At the same time, in order for the charge to be reliably stored and for it to be possible to distinguish between the information to be read, it is necessary to maintain a certain minimum capacitance of the storage capacitor. This minimum capacitance is currently regarded as being approximately 25 fF.
To reduce the space required for a capacitor, it is possible to use a paraelectric material with a high permittivity (high-∈ dielectric material) as the capacitor dielectric. In memory configurations such capacitors are preferably employed as so-called stacked capacitors (the capacitor of the cell is arranged above the associated selection transistor). Memory cells which use paraelectric material as the capacitor dielectric lose their charge when the supply voltage is selected, and thus lose their stored information. Furthermore, owing to the residual leakage current, these cells have to be constantly rewritten (refresh time). By contrast, the use of a ferroelectric material as the capacitor dielectric, owing to the different polarization directions of the ferroelectric material, makes it possible to construct a nonvolatile memory (FRAM), which does not lose its information when the supply voltage is selected and also does not have to be constantly rewritten. The residual leakage current of the cell does not affect the stored signal.
Various high-∈ dielectric materials and ferroelectric materials are known from the literature, for example barium strontium titanate (BST), strontium titanate (ST) or lead zirconium titanate (PZT), as well as ferroelectric and paraelectric polymers, and other materials.
Although these materials do have the desired electrical properties, their significance in practice still remains limited. A principal reason for this is that the materials cannot be readily employed in semiconductor configurations. The materials are produced by means of a sputter-on or deposition process which requires high temperatures in an oxygen-containing atmosphere. Consequently, the conductive materials (e.g. polysilicon, aluminum, or tungsten) used in the semiconductor industry as electrode material are unsuitable, since they are oxidized under such conditions. Therefore, at least the first electrode is usually made from a noble-metal-containing material, such as platinum or ruthenium. However, these new electrode materials are relatively unknown substances in the semiconductor industry and are relatively difficult to apply. A particularly serious problem is that they can only be structured satisfactorily with a thin layer thickness. Furthermore, they are permeable to oxygen, with the result that, during the production of the capacitor dielectric, deep structures become oxidized and satisfactory contact between the first electrode and selection transistor is not guaranteed. Therefore, a barrier which suppresses oxygen diffusion is required beneath the capacitor dielectric.
German published patent application DE 196 40 448 and published International Application WO 98/14992 describe a memory cell of this type, in which the barrier between the first electrode and the connection structure for the selection transistor is formed over the entire surface by nitriding. German published patent application DE 196 40 244 describes a capacitor with a high-∈ dielectric or ferroelectric capacitor dielectric, in which the first electrode comprises an electrode core and a comparatively thin noble-metal-containing layer, and in which the electrode core comprises the material of the connection structure or of the oxidation barrier. This has the advantage that only a thin noble-metal-containing layer has to be structured.
A common feature of all these capacitors having a high-∈ dielectric or ferroelectric capacitor dielectric is that the first electrode is provided in essentially planar configuration.
In U.S. Pat. No. 5,581,436, a thin layer of platinum is applied to the surface of an electrode core, as the first electrode of a capacitor of the type in question. If appropriate, the high-∈ dielectric may be produced as an exposed structure prior to the formation of the first and second electrodes, i.e. the electrodes are then formed on the side walls of the dielectric.
It is accordingly an object of the invention to provide a capacitor with a high-∈ dielectric or ferroelectric capacitor dielectric, which overcomes the above-mentioned disadvantages of the heretofore-known devices of this general type and which leads to a further reduction in the space requirement. With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing a capacitor in a semiconductor configuration on a substrate, which comprises:
forming a sequence of layers on a substrate, the sequence alternately comprising a layer of a first material and a layer of a second material, wherein the first material is selectively etchable with respect to the second material;
etching the sequence of layers to form a layer structure with flanks;
forming a first auxiliary structure of a first auxiliary material covering at least one flank of the layer structure, wherein the first auxiliary structure is selectively etchable with respect to the second material;
forming a second auxiliary structure covering at least one other flank of the layer structure and mechanically connecting the layers of the second material;
covering a substrate surface surrounding the layer structure with a filler layer up to a top edge of the layer structure;
removing the layers of the first material and the first auxiliary structure selectively with respect to the layers of the second material and with respect to the second auxiliary structure;
filling a cavity formed in the removing step with a noble-metal-containing electrode material and forming a first electrode with lamellae in the cavity formed by the layers made from the first material and with a support structure connecting the lamellae in the cavity formed by the first auxiliary structure;
removing the layers of the second material and the second auxiliary structure selectively with respect to the electrode material;
conformally applying a capacitor dielectric of a material selected from the group consisting of high-∈ dielectric and ferroelectric material to an exposed surface of the first electrode; and
forming a second electrode on the capacitor dielectric.
In accordance with an added feature of the invention, the first material is undoped, n-doped, and pxe2x88x92-doped polysilicon, and the second material is p+-doped polysilicon.
In accordance with an additional feature of the invention, the first auxiliary structure is formed on two flanks of the layer structure which lie opposite one another in a first direction.
In accordance with another feature of the invention, the second auxiliary structure is formed on two flanks of the layer structure which lie opposite one another in a second direction.
In accordance with a further feature of the invention, the first auxiliary structure and/or the second auxiliary structure are formed by selective silicon deposition.
In accordance with again an added feature of the invention, the second auxiliary structure is formed by inclined implantation with p-doping ions into the flank of the layer structure.
In accordance with again an additional feature of the invention, after the cavity is filled with the electrode material, the electrode material deposited on the filler layer is removed with a CMP process.
In accordance with again another feature of the invention, subsequently to the formation of the first electrode, the filler layer is removed selectively with respect to the electrode material and with respect to the substrate surface.
In accordance with again a further feature of the invention, the substrate is provided with an insulating layer on a surface facing toward the sequence of layers and forming the sequence of layers on the insulating layer, wherein the insulating layer has a contact arranged therein comprising a diffusion barrier connected to the first electrode.
In accordance with a concomitant feature of the invention, the process is directed to producing a multiplicity of capacitors in a linear arrangement along a second direction on the substrate. The following production steps are thereby performed:
structuring the sequence of layers to strip-shaped layer structures running in the second direction, with the first auxiliary structures formed on the flanks of the structures;
structuring the strip-like layer structures, together with the first auxiliary structures, to form a multiplicity of island-like layer structures;
forming the second auxiliary structures on the flanks of each island opposite one another in the second direction; and
filling an entire space between the islands with the filler layer.
With the above and other objects in view there is also provided, in accordance with the invention, a capacitor in a semiconductor configuration on a substrate, comprising:
a noble-metal-containing first capacitor electrode formed with a plurality of mutually spaced-apart lamellae, the lamellae being oriented substantially parallel to the substrate surface;
a support structure mechanically and electrically connecting the lamellae to one another on a flank thereof;
a capacitor dielectric formed of a material selected from the group consisting of high-∈ dielectric and ferroelectric material disposed on the first capacitor electrode; and
a second capacitor electrode on the capacitor dielectric.
In accordance with yet an added feature of the invention, the lamellae define two mutually opposite flanks and the support structure is arranged on the two flanks of the lamellae.
In accordance with yet an additional feature of the invention, the substrate, on a surface thereof facing toward the lamellae, is formed with an insulating layer having a contact arranged therein, the contact comprising a diffusion barrier and being connected to the first capacitor electrode.
In accordance with a concomitant feature of the invention, the substrate contains a MOS transistor and the contact connects a source-drain region of the transistor to the first electrode.
The invention may thus be summarized as follows: the first electrode comprises at least two lamellae which are spaced apart from one another, lie essentially parallel to the substrate surface and are connected to one another via a support structure. The surface which acts as a capacitor is consequently significantly larger than the substrate surface claimed. The support structure may in particular be arranged on one outer flank of the lamellae or on two opposite outer flanks.
The geometric structure of the first electrode corresponds to the form of so-called xe2x80x9cfin stack capacitors,xe2x80x9d which comprise doped polysilicon. During the production of these known capacitors, polysilicon with a high layer thickness (composed of a plurality of individual layers) has to be anisotropically etched, as described, for example, in the published European patent applications EP 756 326 A1 and EP 779 656 A2. This geometric structure does not appear to be achievable for noble-metal-containing electrodes, in view of the difficulty in etching such electrodes.
The production process according to the invention makes it possible to produce a capacitor with a noble-metal-containing first electrode based on the fin stack principle without there being any need for anisotropic etching of the electrode material. It is based on using a body which preferably comprises p+-polysilicon, which is similar to that of the fin stack capacitor, as a negative mold for a metal body which fulfills the specific requirements imposed on electrodes when using high-∈ dielectric materials and ferroelectric materials.
To produce the first electrode, a sequence of layers which alternately has a layer made from a first material and a layer made from a second material, it being possible to etch the first material selectively with respect to the second material, is formed on a substrate which may contain an insulating layer with a connection embedded therein. The sequence of layers is then etched to form a layer structure with flanks. A first auxiliary structure is then formed on at least one flank, which structure comprises a first auxiliary material which, like the first material, can be etched selectively with respect to the second material. At least one flank is not covered by the first auxiliary structure. A second auxiliary structure is then formed, which covers at least one, and preferably all the remaining external flanks of the sequence of layers and mechanically connects the layers made from the second material. The substrate surface which surrounds this arrangement is covered with a filler layer all the way to the top edge of the layer structure. Then, the layers made from the first material and the first auxiliary structure are removed selectively with respect to the layers made from the second material and with respect to the second auxiliary structure. Thus, the filler layer and the layers made from the second material which are connected to one another by the second auxiliary structure then remain on the substrate. This structure forms a negative mold with a cavity which has been formed by the removal of the layers made from the first material and of the first auxiliary structure. The cavity is filled, all the way up to the top edge of the filler layer, with a noble-metal-containing electrode material, in order to form a first electrode. Then, the layers made from the second material and the second auxiliary structure are removed selectively with respect to the electrode material, and preferably the filler layer is also removed, in particular selectively with respect to the substrate surface. This results in a first electrode with an exposed surface, to which a capacitor dielectric made from a high-∈ dielectric material or a ferroelectric material is then conformally applied. Finally, the second electrode is formed on the capacitor dielectric.
There are numerous possibilities for the selection of the first and second material and for the arrangement and formation of the first and second auxiliary structures. Neither the layers made from the first or second material nor the auxiliary structures remain in the finished semiconductor arrangement. They can therefore be selected according to process-engineering criteria; the electrical properties are unimportant. The first auxiliary structure preferably comprises the first material, and the second auxiliary structure preferably comprises the second material.
A suitable material for the first electrode is, in particular, platinum, although ruthenium oxide and other noble-metal-containing materials which are known for use in a high-∈ or ferroelectric capacitor and which can be applied conformally in a cavity, for example by means of MOCVD, are also suitable. The second electrode preferably comprises the same material as the first, but may also be formed from another suitable material, such as W or TiN, another metal or doped polysilicon. The second electrode of the capacitor is separated from the first electrode by a high-∈ dielectric material or a ferroelectric material.
The substrate may contain a connection for the first electrode, in which case the rest of the substrate surface is covered with an insulating layer. The noble-metal-containing first electrode then covers part of the substrate surface and covers over this connection, so that electrical contact is guaranteed.
Preferably, the capacitor is used in a DRAM cell. The substrate then contains the associated MOS selection transistor. An S/D region of the transistor is connected to the first electrode via the connection which has been explained above. The connection preferably has a conductive oxygen barrier (for example titanium nitride) in its upper region and otherwise comprises, for example, titanium, polysilicon, tungsten or the like.
In a preferred embodiment of the production process, the layers made from the first material comprise pxe2x88x92-doped polysilicon or undoped polysilicon and the layers made from the second material comprise p+-doped polysilicon. The first auxiliary structure may then be formed by selective silicon deposition of undoped or n- or pxe2x88x92-doped polysilicon. The epitaxial or selectively deposited layer is then removed again on preferably two mutually opposite external flanks of the layer structure. Alternatively, particularly if it is chosen to use different first and second materials, the first auxiliary structure may also be formed as a spacer or by sidewall implantation into one or two of the layer structures. In the abovementioned layer system, the second auxiliary structure is preferably formed by lateral implantation with boron ions. Alternatively, instead of a lateral boron implantation, it is possible to carry out a selective Si deposition of p+-doped polysilicon. If both auxiliary structures are formed using epitaxy, the sequence of the selective p+- and pxe2x88x92-deposition may also be reversed, i.e. the second auxiliary structure may be produced before the first auxiliary structure.
The filler layer is preferably formed by deposition of a thick layer of silicon oxide, which is subsequently ground down (CMP) to the height of the layer structure, i.e. until the first auxiliary structure or the top layer made from the first material is at least partially exposed.
Although it can be employed very generally, the capacitor and the production process are described below with reference to the example of a DRAM cell configuration.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a capacitor with a high-∈ dielectric material or a ferroelectric material based on the fin stack principle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.